Select the marquee tool and press to open the settings.

Choose 'Rectangle' from the 'Shape' menu.

When you are finished with these settings, press the 'Done' button.

Press and hold the spacebar or to begin selecting your region.

Use to move the cursor creating a box to select your desired region.

When you are finished release the spacebar or setting the selected region.

For more information on what you can do with the marquee tool, View The Tutorial

When you are ready to apply the marquee, press , then select 'Edit', then 'Deselect'.